The alveolar epithelium separates gas and liquids in the lung and allows gas diffusion. A property of the tissue which is important to this barrier function is its passive permeability to solute. This permeability is not static but dynamic, and increases at high lung inflation. Utilizing multiple radiolabeled solute transfer techniques developed for the adult lung, this project will define the elasticity and the reversibility of inflation-induced permeability changes, and differences in permeability changes produced by static inflation and ventilation in in vivo adult dog lungs. Similar techniques in the same animal will be used to determine if the adult alveolar epithelium actively transport electrolytes as does a fetal lung. This is a step toward understanding more of the function of the alveolar epithelium. The second major area will be to study abnormal epithelial permeability in immature and fetuses during the first hours of ventilation. The alveolar epithelium of a ventilated immature fetus permits free diffusion of all solutes between fetal lung liquid and blood. This project will compare the absorption of fetal lung liquid and effectiveness of ventilation of corticosteroid pre-treated immature lambs, immature lambs with acute exogenous addition of surface active substances to the fetal lung liquid, and control immature lambs. In addition the project will define the effects of abnormal alveolar permeability produced by hyperinflation of the lungs during the first breaths on lung liquid absorption and establishment of ventilation in mature fetal lambs.